The present invention relates to a driving force distribution control system for controlling a driving force distribution between front and rear wheels of a four wheel drive vehicle, and more specifically to a driving force distribution control system for improving a controllability of a vehicle near the cornering limits of the vehicle.
Japanese Patent provisional publication 63-203421 and a U. S. Pat. No. 5,060,747, for example, disclose driving torque distribution control systems designed to control a front wheel driving torque as a function of a front and rear wheel speed difference by using a control gain which is decreased as the lateral acceleration of the vehicle increases.
However, this conventional system is not completely satisfactory in some cases. When, for example, the front and rear wheel speed difference is increased by a driver's operation on the accelerator during a high lateral acceleration turning, these conventional systems increase the front wheel driving torque. This increase of the front wheel driving torque results in a decrease in the front wheel side force, and hence an increase in the understeer tendency. When the driver depresses the accelerator pedal toward a clipping point in a middle period B in a high lateral acceleration turning motion (as shown in FIG. 9), this conventional control system responds to this driver's accelerator operation by increasing the front wheel driving torque, and as a result, increases the understeer of the vehicle due to a decrease of the front wheel side force. Therefore, the course of the cornering motion of the vehicle deviates outwards (as shown in FIG. 9), and the vehicle cannot reach the target clipping point.